1. Field of the Invention
The present invention relates to a welding gun driving device including a pressure rod axially moved forward and backward via a feed screw mechanism by an electric motor.
2. Background Art
Conventionally, as a drive device of this type, there has been known a device in which a rotor of an electric motor is formed into a hollow shape through which a pressure rod is insertable, in which a feed screw mechanism is formed by: a screw shaft fixed to the rotor concentrically with this rotor; and a nut portion screwed to the screw shaft, and in which the pressure rod is formed by: the nut portion; and a hollow rod portion extending from the nut portion to the axial front (see Patent Document 1, for example). The turning of the pressure rod is prevented, and therefore, the turning of the nut portion is also prevented, thereby allowing the pressure rod to be axially moved forward and backward via the nut portion due to the rotation of the screw shaft. It should be noted that, in this device, a guide sleeve, having a flange portion fixed to an outer face of a front end portion of a motor housing for the electric motor, is inserted into the rotor with a clearance existing therein, and the pressure rod is axially slidably inserted and supported through this guide sleeve. [Patent Document 1] JP-A-2001-150147
Further, it should be noted that in the case of an X-type welding gun, a pressure rod is connected to a gun arm, and the gun arm is closed due to the movement of the pressure rod toward the axial front, thereby pressurizing a work. On the other hand, in the case of a C-type welding gun, a movable electrode tip is connected to a pressure rod, and the movable electrode tip abuts against a work due to the movement of the pressure rod toward the axial front, thereby pressurizing the work.
In the conventional example (JP-A-2001-150147), a front end portion of the rotor is pivotally supported by a bearing placed on an inner periphery of the front end portion of the motor housing. In this example, in order to ensure the supporting rigidity for the motor, the thickness of the front end portion of the rotor supported by the bearing cannot be drastically reduced. Furthermore, since the guide sleeve is provided with the clearance existing at the inner periphery of the rotor, the inner diameter of the guide sleeve is forced to be reduced. As a result, the outer diameter of the pressure rod, inserted and supported through the guide sleeve, is also reduced, and the buckling strength of the pressure rod is likely to be deficient. Moreover, if the outer diameter of the pressure rod is increased so as to obtain a sufficient buckling strength thereof, the electric motor is increased in size with a large diameter.
Further, in the device of the above-described conventional example (JP-A-2001-150147) includes a guide portion through which the pressure rod is axially slidably inserted and supported. Furthermore, the pressure rod is formed to have an equal diameter along its entire length, and the pressure rod is supported by the guide portion along the length corresponding to the axial length of the guide portion.
In this device, it is desired that the axial length of the guide portion be set to a sufficient length so as to obtain the supporting rigidity required for the pressure rod during a high load period when the pressure rod is moved to an axial front stroke end position to pressurize a work. However, in the above-described example, the length of the guide portion for supporting the pressure rod does not change even during a low load period when the pressure rod is retracted to the axial rear, and if the axial length of the guide portion is increased, the sliding resistance of the pressure rod against the guide portion is increased, thereby increasing energy loss.
Further, in order to smoothly operate the drive device, an outer peripheral face of the screw shaft needs to be lubricated with a lubricant. However, in the above-described conventional example (JP-A-2001-150147), the screw shaft is covered by the rotor and the pressure rod, and a lubricant cannot be supplied to the outer peripheral face of the screw shaft from outside. Therefore, the drive device has to be regularly disassembled to supply a lubricant to the outer peripheral face of the screw shaft, and a welding gun cannot be used during this time period, thus causing a decrease in the rate of operation of the welding gun.